A known technique of operating electronic components at cryogenic temperatures is to mount the components on a so-called "cold finger" within an evacuated chamber. A problem can arise in making electrical connections between the exterior of the enclosure, which may be at ambient temperature, and the electronic components, which are at cryogenic temperatures, in that electrically conductive signal paths also provide thermally conductive paths. The more connecting paths, the greater the conducted heat and the heavier the loading on the cryogenic cooler. This is a particular problem with radio-frequency signal paths where conductor dimensions are determined by the need to maintain a good impedance match and may involve the provision of a conducting ground plane. The coupling paths also need to allow for the thermal expansion and contraction which may arise due to the thermally-produced movement between the components and the enclosure which occurs when the assembly is being brought to its cryogenic operating temperature or returns to ambient temperature when the apparatus is shut down.
In one known approach, each path comprises a length of coaxial cable extending between the walls of the enclosure and the cryogenic components. However this known arrangement suffers the disadvantage that, in order to minimise heat loss by conduction, the cable must be made relatively long. While this arrangement has proved satisfactory where a small number of signal paths is concerned, for large numbers of paths, the large number of parallel paths provides a correspondingly reduced thermal resistance, leading to a proportional increase in heat load. Another disadvantage is that coaxial cable will inevitably contain a certain amount of trapped gas which will progressively leak into the vacuum space over a period of time. This gas will provide another mechanism for heat transfer by convection or conduction between ambient and the cold component. The longer the cable, the greater the potential for out gassing of trapped gases. The user then has the inconveniences of having to periodically re-pump the enclosure to restore vacuum.
Another problem which can arise concerns adjustment of adjustable components such as tuning screws which may be necessary when setting up a circuit operating at cryogenic temperatures.
The present invention seeks to ameliorate the above disadvantages of the prior art.